title: Regulatory roles for Tiam1, a guanine nucleotide exchange factor for Rac1, in glucose-stimulated insulin secretion in pancreatic beta-cells.
abstract: Using various biochemical, pharmacological and molecular biological approaches, we have recently reported regulatory roles for Rac1, a small G-protein, in glucose-stimulated insulin secretion (GSIS). However, little is understood with respect to localization of, and regulation by, specific regulatory factors of Rac1 in GSIS. Herein, we investigated regulatory roles for Tiam1, a specific nucleotide exchange factor (GEF) for Rac1, in GSIS in pancreatic beta-cells. Western blot analysis indicated that Tiam1 is predominantly cytosolic in distribution. NSC23766, a specific inhibitor of Tiam1-mediated activation of Rac1, markedly attenuated glucose-induced, but not KCl-induced insulin secretion in INS 832/13 cells and normal rat islets. Further, NSC23766 significantly reduced glucose-induced activation (i.e. GTP-bound form) and membrane association of Rac1 in INS 832/13 cells and rat islets. Moreover, siRNA-mediated knock-down of Tiam1 markedly inhibited glucose-induced membrane trafficking and activation of Rac1 in INS 832/13 cells. Interestingly, however, in contrast to the inhibitory effects of NSC23766, Tiam1 gene depletion potentiated GSIS in these cells; such a potentiation of GSIS was sensitive to extracellular calcium. Together, our studies present the first evidence for a regulatory role for Tiam1/Rac1-sensitive signaling step in GSIS. They also provide evidence for the existence of a potential Rac1/Tiam1-independent, but calcium-sensitive component for GSIS in these cells.
authors.affiliation: Department of Pharmaceutical Sciences, Eugene Applebaum College of Pharmacy and Health Sciences, Wayne State University, Detroit, MI 48201, United States.
authors.forename: Rajakrishnan
authors.initials: R
authors.lastname: Veluthakal
authors.forename: Suresh Vasu
authors.initials: SV
authors.lastname: Madathilparambil
authors.forename: Phillip
authors.initials: P
authors.lastname: McDonald
authors.forename: Lawrence Karl
authors.initials: LK
authors.lastname: Olson
authors.forename: Anjaneyulu
authors.initials: A
authors.lastname: Kowluru
mesh_terms: D000634:Aminoquinolines / Q000494:pharmacology; D000818:Animals; D002478:Cells, Cultured; D005947:Glucose / Q000502:physiology*; D020662:Guanine Nucleotide Exchange Factors / Q000037:antagonists & inhibitors / Q000502:physiology*; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D050417:Insulin-Secreting Cells / Q000187:drug effects / Q000378:metabolism*; D009363:Neoplasm Proteins / Q000037:antagonists & inhibitors / Q000502:physiology*; D011743:Pyrimidines / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D000075684:T-Lymphoma Invasion and Metastasis-inducing Protein 1; D020830:rac1 GTP-Binding Protein / Q000037:antagonists & inhibitors / Q000502:physiology*
chemical_list: D000634:Aminoquinolines; D020662:Guanine Nucleotide Exchange Factors; D007328:Insulin; C487513:NSC 23766; D009363:Neoplasm Proteins; D011743:Pyrimidines; D000075684:T-Lymphoma Invasion and Metastasis-inducing Protein 1; C513259:Tiam1 protein, rat; C499134:Rac1 protein, rat; D020830:rac1 GTP-Binding Protein; D005947:Glucose